Closed-cycle thermochemical decomposition of water has for a long period of time been a subject of intensive investigation. There are thermodynamic limitations which have thwarted the development of a simple two-step process consisting of one heat absorbing, i.e. hot, step and one heat rejecting, i.e. cool, step. The minimum temperature difference in .degree.C between the hot and cool steps (T.sub.hot - T.sub.cool) for any process which accomplishes reaction (1) is given by equation (2). EQU H.sub.2 0 .fwdarw. H.sub.2 + 1/2 O.sub.2 ( 1) EQU t.sub.hot - T.sub.cool = 58,000/.DELTA. S (2)
where 58,000 is the heat of formation of water in calories per mole, and .DELTA.S is the standard entropy change of the process. The direct thermolysis of water, reaction (1), has entropy change, .DELTA.S, of only 40 entropy units (e.u.) and would require a minimum temperature swing of 1450.degree.C, which is too large for practical processes at present. Real processes must be limited to smaller temperature swings, which would require an entropy change of 70 to 80 e.u., a value which has been considered impossible to achieve in one single reaction. All previously proposed processes for practical closed-cycle water thermolysis approach this problem by using a sequence of several reactions, apportioning the 70 to 80 e.u. total change among the multiple reaction steps. Many of these multireaction thermolysis schemes also involve highly corrosive chemicals, requiring special materials for construction. The present invention, however, comprises the discovery of a single reaction which does have the required large entropy change, and can therefore accomplish the thermolysis of water in a simple two-step process described above using practical temperature swings.